In a conventional smart card which is in widespread use, for example, in the electronic payment transactions sector, the communication between the chip located on the smart card and a read device is contact-based, i.e. via smart card contacts exposed towards the outside of the smart card.
For this purpose, however, for use the smart card always needs to be singled out and inserted into a corresponding read device, which a user may find objectionable. A development which solves this problem provides so-called dual-interface smart cards, in which the chip, in addition to the conventional contact-based interface, can also communicate by means of a contactless interface. The contactless interface on the smart card can have a smart card antenna, which is contained in the smart card and is connected to the chip. The smart card antenna and the chip can be arranged together on a smart card module, in which case such a miniaturized form of the smart card antenna can then be referred to as a smart card module antenna. The joint arrangement of the coil and the chip on a smart card module is also referred to as CoM (coil on module). Irrespective of the type of smart card antenna, a galvanic connection is formed between said smart card antenna and the smart card module or the chip.
In an electronic payment system, for example, a functional distance of up to 4 cm between the chip and the read unit is required. However, meeting this setpoint input can prove to be problematic since, under certain circumstances, it is not possible for a sufficiently large smart card module antenna for enabling wireless communication at the required distance to be arranged on the small area which is available on the smart card module. In order to improve the performance of the contactless communication, in addition a so-called amplifier antenna (also referred to as booster antenna) can be built into a smart card and inductively coupled to the smart card module or the smart card module antenna arranged on the smart card module. Likewise, such a booster antenna can be inductively coupled to the CoM of a purely contactless smart card for improving the performance of the contactless communication. The booster antenna can be provided on a separate layer and contained in the smart card. The separate layer which contains the booster antenna may be or may have been laminated into the smart card during manufacture of the smart card, for example. The possible read or write distance between a write or read device and the smart card module is substantially increased by the booster antenna.
FIG. 1 shows a contactless smart card 100 with a smart card body 102, a booster antenna 104 integrated (for example laminated) therein and a contactless smart card module arrangement (for example also referred to as coil on module, CoM) 106, wherein the booster antenna 104 partially surrounds the contactless smart card module arrangement 106.
The booster antenna 104 is formed by a large ring-shaped conductor loop 108, wherein a small part of the conductor loop is formed to give a small conductor loop 110 which partially surrounds the contactless smart card module arrangement 106, for example a coil on module 106, which is arranged within a peripheral region of the large conductor loop 108.
FIG. 2 shows an associated (simplified) equivalent circuit diagram 200 illustrating that the conventional booster antenna 104 is a simple series resonant circuit 202. Said simple series resonant circuit includes an inductance 204 of the large conductor loop 108 (Llarge), a capacitance 206 of the large conductor loop 108 (Clarge), an ohmic resistance 208 of the large conductor loop 108 (Rlarge), and an inductance 210 of the small conductor loop 110 (Lsmall). A first inductive coupling (with corresponding coupling factor k) 212 to a read device (reader) (not illustrated in FIG. 2) takes place via the inductance 204 of the large conductor loop 108 (Llarge); a second inductive coupling (k) 214 to the contactless smart card module arrangement 106, for example an on-chip antenna (OCA) or a chip on module (CoM), takes place via the inductance 210 of the small conductor loop 110 (Lsmall).
A conventional booster antenna is generally a simple series resonant circuit. In this case, a large conductor loop is used for inductively coupling in energy. In order to achieve a booster effect (amplifier effect), a small part of this conductor loop is formed to give a further, small conductor loop, which surrounds a CoM. The geometric approximation of the small conductor loop and the CoM results in good coupling between these components. The nearer and more similar the small conductor loop of the booster antenna and the antenna of the CoM are with respect to one another, the better the coupling factor thereof is.
A booster antenna of this type is, however, complicated in terms of manufacture and difficult to verify. Furthermore, parameterization of the so-called loading effect plays a significant role in the design process.
The writing and reading of very small CoMs requires high field intensities and/or small distances between the write/read device and the smart card module even when using a conventional booster antenna, however.